linux-kernel - Re: [PATCH] driver/pci: reduce the single block time in pci_read

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Date:   Wed, 9 Sep 2020 20:25:43 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Jiang Biao <benbjiang@...il.com>
Cc:     bhelgaas@...gle.com, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, Jiang Biao <benbjiang@...cent.com>,
        Bin Lai <robinlai@...cent.com>
Subject: Re: [PATCH] driver/pci: reduce the single block time in
 pci_read_config

On Mon, Aug 24, 2020 at 01:20:25PM +0800, Jiang Biao wrote:
> From: Jiang Biao <benbjiang@...cent.com>
> 
> pci_read_config() could block several ms in kernel space, mainly
> caused by the while loop to call pci_user_read_config_dword().
> Singel pci_user_read_config_dword() loop could consume 130us+,
>               |    pci_user_read_config_dword() {
>               |      _raw_spin_lock_irq() {
> ! 136.698 us  |        native_queued_spin_lock_slowpath();
> ! 137.582 us  |      }
>               |      pci_read() {
>               |        raw_pci_read() {
>               |          pci_conf1_read() {
>   0.230 us    |            _raw_spin_lock_irqsave();
>   0.035 us    |            _raw_spin_unlock_irqrestore();
>   8.476 us    |          }
>   8.790 us    |        }
>   9.091 us    |      }
> ! 147.263 us  |    }
> and dozens of the loop could consume ms+.
> 
> If we execute some lspci commands concurrently, ms+ scheduling
> latency could be detected.
> 
> Add scheduling chance in the loop to improve the latency.

Thanks for the patch, this makes a lot of sense.

Shouldn't we do the same in pci_write_config()?

> Reported-by: Bin Lai <robinlai@...cent.com>
> Signed-off-by: Jiang Biao <benbjiang@...cent.com>
> ---
>  drivers/pci/pci-sysfs.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/pci/pci-sysfs.c b/drivers/pci/pci-sysfs.c
> index 6d78df9..3b9f63d 100644
> --- a/drivers/pci/pci-sysfs.c
> +++ b/drivers/pci/pci-sysfs.c
> @@ -708,6 +708,7 @@ static ssize_t pci_read_config(struct file *filp, struct kobject *kobj,
>  		data[off - init_off + 3] = (val >> 24) & 0xff;
>  		off += 4;
>  		size -= 4;
> +		cond_resched();
>  	}
>  
>  	if (size >= 2) {
> -- 
> 1.8.3.1
>